The 32-year-old Höwedes left Lokomotiv Moscow last month through mutual consent when the Russian league was suspended in the midst of the coronavirus pandemic.
Höwedes told German magazine Der Spiegel that after spending time on holiday with his young son, soccer seemed "suddenly too unimportant" for him, and that he had struggled to be away from his family.
Between 2011-17, Höwedes played 44 times for the German national team and at the 2014 World Cup in Brazil he played every minute for his side.
He came through the Schalke youth system and played for the club professionally from 2007-18, including an injury-stricken loan period at Juventus in 2017-18. In 2018, Höwedes moved in with Lokomotiv to Russia.